Sedimentation tanks – effective flow through period:\r Compared with the theoretical detention period, the effective flow through period of a sedimentation tank under real flow conditions is generally

Introduction / Context:
Clarifiers and settling basins are rarely ideal plug flow reactors. Short circuiting, dead zones, and mixing reduce the effective time available for particle settling compared with the theoretical detention period computed from volume and flow alone. Understanding the practical flow through period is essential for realistic performance expectations.

Given Data / Assumptions:

• Theoretical detention period t_d = basin volume / flow rate.
• Displacement efficiency E_d = flow through period / detention period, where 0 < E_d < 1 for real tanks.
• Hydraulic inefficiencies exist due to inlet jets, outlet draw, and geometry.

Concept / Approach:
Because E_d is less than 1, the effective flow through period is t_f = E_d * t_d, which is less than t_d. Designers mitigate this with inlet baffling, flocculation ahead of sedimentation, appropriate aspect ratios, and properly designed effluent launders to improve residence time distribution.

Step-by-Step Reasoning:
Start with t_d from volume and flow.Account for non ideal mixing: t_f = E_d * t_d.Since E_d is less than 1, conclude t_f is less than t_d.

Verification / Alternative check:
Tracer tests routinely show mean residence times less than nominal detention, confirming the reduction and guiding retrofit measures like baffle walls and density current control.

Why Other Options Are Wrong:

• Equal or more than detention is inconsistent with typical non idealities.
• Dividing by displacement efficiency would increase time, contrary to observed behavior where E_d < 1.

Common Pitfalls:

• Assuming textbook detention ensures removal efficiencies without examining hydraulics.
• Ignoring seasonal temperature stratification that worsens short circuiting.

Final Answer:
Less than the detention period